SCEC Award Number 16123 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Reconciling supershear transition of dynamic ruptures with low fault prestress and implications for the San Andreas Fault
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
Other Participants Prof. Ares Rosakis, research scientist Dr. Vito Rubino
SCEC Priorities 3e, 4b, 4d SCEC Groups SoSAFE, GMP, FARM
Report Due Date 03/15/2017 Date Report Submitted 05/10/2017
Project Abstract
 Supershear rupture propagation on a uniform fault requires a high level of shear stress compared to dy-namic strength, as indicated by analytical and experimental results. Yet many observations suggest that mature strike-slip faults that host large earthquakes operate at low overall levels of shear prestress. The apparent incompatibility between supershear propagation and low level of shear prestress can be re-solved on heterogeneous faults. In simulations, the stress field of the main rupture dynamically triggers a secondary crack at a location of favorable heterogeneity - a weak spot or a nucleating zone - which transitions to supershear speed under a wide range of conditions. Our goal is to study this phenomenon experimentally. To design experiments with appropriate heterogeneity, a key ingredient is the evolution of dynamic friction. In this SCEC project, we have developed a unique experimental technique for full-field imaging of dynamic ruptures and used this technique to study friction evolution during spontaneously developing dynamic rupture. Our main finding is that the measured friction is consistent with rate-and-state friction formulations supplemented with flash heating at high slip rates. The resulting shear stress vs. slip behavior highly varies between different experiments and depends on the specifics of the dynamic rupture. The quantification of the dynamic friction has enabled us to design appropriate experiments that we are in the process of performing. The new imaging technique allows us to capture the development of strains and stresses at the favorable heterogeneity during its rupture.
Intellectual Merit Can supershear earthquakes occur under overall low level of applied shear prestress? For a rupture to transition to supershear speed on a uniform fault, a high level of shear stress is required, as indicated by theoretical and numerical studies as well as laboratory experiments. Yet observations and simulations indicate that well-developed, mature strike-slip faults that host large earthquakes operate at low overall levels of shear prestress. It is important to understand whether low-stressed faults can generate su-pershear ruptures since supershear rupture can cause much larger shaking far from the fault than sub-Rayleigh ruptures. This is of particular relevance to the Southern San Andreas Fault which is locked and loaded for the next large earthquake. Our goal is to design and conduct laboratory experiments that study supershear transition by dynamic triggering of a favorable patch, which can occur under much lower overall levels of prestress, as established in numerical models. Our numerical simulations show that proper quantification of dynamic friction evolution is key to designing suitable experiments; using the same linear slip-weakening law for different prestresses results in numerical predictions that do not correspond well to the actual subsequent experiments. We have developed a unique experimental tech-nique that enables us to quantify full fields of dynamic displacements, strains, and stresses. This tech-nique combines ultra high-speed photography with digital image correlation to compute full-field dy-namic displacements, particle velocities, strains and stresses. This technique has allowed us to measure evolution of friction properties during spontaneous dynamic ruptures over a broad range of slip rates. Using the experimentally derived friction properties, we have determined the range of experimental parameters that would enable us to observe supershear transition under low fault prestress according to the prior numerical studies. We are currently in the process of performing these experiments.
Broader Impacts Understanding the range of potential realistic scenarios on San Andreas and other mature strike-slip faults is crucially important for the estimates of seismic hazard and ground motion. This project aims to study whether supershear earthquakes can occur on faults with low prestress, if suitable patches of heterogeneities are present. The effects of such occurrence on the shaking in Southern California can then be explored in large-scale simulations. A research scientist and a student have gained valuable research experience by participating in the project and interacting with the SCEC community. They have also participated in several outreach activities oriented towards sixth to ninth graders of the Los Angeles area.
Exemplary Figure Figure 3. Quantifying rupture evolution in laboratory experiments and steady-state measurements of friction. (a) Slip rate and (b) shear stress time histories for an experiment exhibiting a sub-Rayleigh pulse-like rupture propagating first, followed by a supershear crack-like rupture induced by slip interac-tion with the sample boundary (experimental conditions P = 12 MPa and α = 24°). The measurements are obtained using high-speed photography and DIC techniques. (c) Experimental measurements of steady-state friction coefficient for sustained slip at a given slip rate, based on multiple ruptures with different prestress conditions (all colored symbols except for green dots). Green dots are low-velocity measurements obtained in collaboration with Drs. Kilgore, Beeler, and Lu in a different apparatus and reported in Lu (2009). Fits to low-slip data with the standard rate-and-state friction formulation (green curve) and all data points with the combined formulation of rate-and-state friction enhanced by flash heating (black curve) demonstrate that our steady-state measurements are consistent with the combined formulation. (d) Experimental measurements of dynamic friction on quartzite samples (Goldsby and Tullis, 2011), showing qualitatively similar behavior for rocks. Note the different horizontal scale for the two plots. Modified from Rubino, V., A.J. Rosakis and N. Lapusta, Understanding dynamic friction through spontaneously evolving laboratory earthquakes, Nature Communication, accepted for publication, 2017.